Due to recent advancement of information communication industry and digitalization of electronic devices, image sensors with improved performance are being used in various fields such as digital cameras, camcorders, mobile phones, Personal Communication Systems (PCS), game machines, security cameras, and medical micro cameras. Generally, an image sensor includes a pixel area including a photodiode and a peripheral circuit area. A unit pixel includes a photodiode and a transfer transistor. As the resolution of the image sensors increases and the size of unit pixels becomes smaller, there has been a need to reduce the physical volume of the photodiode. Accordingly, the capability for generating photoelectron in the photodiode becomes important.